9939 F. M. BALFOUR, 
the cases (whose importance has been recently insisted upori 
by Professor Huxley), of the Asteroids, the Echinoids, Sa- 
gitta, and others, in which the body cavity arises as an out- 
growth of the alimentary canal and the somatopleure and 
splanchnopleure are formed from that outgrowth, it is clear 
without further remark that the mesoblast is derived from 
the hypoblast. For the echinoderms in which the water-vascu- 
lar system and muscular system arise as a solid outgrowth of 
the wall of the alimentary canal there can also be no question 
as to the derivation of the mesoblast from the hypoblast. 
Amongst other worms, in addition to Sagitta, the investi- 
gations of Kowalevsky seem to show that in Lumbricus the 
mesoblast is derived from the hypoblast. 
Amongst Crustaceans, Bobretzky’s! observations on Oniscus 
(‘Zeitschrift fur wiss. Zoologie,’ 1874) lead to the same con- 
clusion. 
In insects Kowalevsky’s observations lead to the conclusion 
that mesoblast and hypoblast arise from a common mass of 
cells; Ulianin’s observations bring out the same result for 
the abnormal Poduride, and Metschnikoff’s observations 
show that this also holds for Myriapods. 
In mollusks the point is not so clear. 
In Tunicates, even if we are not to include them amongst 
vertebrates,? the derivation of mesoblast from hypoblast is 
without doubt. 
Without going further into details it is quite clear that 
the derivation of the mesoblast from the hypoblast is very 
general amongst invertebrates. 
It will hardly be disputed that primitively the muscular 
system of the body wall could not have been derived from 
the layer of cells which lines the alimentary canal. We see 
indeed in Hydra and the Hydrozoa that in its primitive dif- 
ferentiation, as could have been anticipated beforehand, the 
muscular system of the body is derived from the epiblast cells. 
What, then, is the explanation of the widespread derivation 
of the mesoblast, including the muscular system of the body, 
from the hypoblast ? 
The explanation of it may, I think, possibly be found, and 
at all events the suggestion seems to me sufficiently plausible 
to be worth making, in the fact that in many cases, and 
probably this applies to the ancestors of the vertebrates, 
the body cavity was primitively a part of the alimentary. 
1 He says, p. 182: “‘ Bevor aber die Halfte der Hioberflache von den Em- 
bryonalzellen bedeckt ist, kommt die erste gemeinsame Anlage des mittleren 
und unteren Keimblattes zum Vorschein.” 
2 Anton Dohrn, ‘ Der Ursprung des Wirbelthieres.’ Leipzig, 1875. 
